1. Field of the Invention
The present invention relates to a pulse code modulation circuit for converting an analog signal into a digital signal and pulse-code modulating this digital signal in accordance with a predetermined rule coding.
2. Description of the Prior Art
A pulse code modulation circuit is known which converts an analog signal into a digital signal by using a parallel A/D converter and then applies pulse-code modulation (hereinafter abbreviated as PCM) to this digital signal in accordance with a predetermined rule, as disclosed, for example, in "A Video-Rate 10-bit Two-step Parallel A/D Converter" by H. Kimura, et al in the technical research report of the Institute of Electronic Information and Communication Engineers of Japan, ICD 90-55-63, Jun. 22, 1990, pp 57, FIG. 1.
In the pulse-code modulation circuit, there is often desired to alter the rule of PCM in accordance with the level of an analog signal or the level of a digital signal as converted or in accordance with the result of application of the PCM. In the conventional pulse-code modulation circuit, however, a predetermined rule of the PCM is stored in a read only memory (ROM), so that it is not possible to freely rewrite the content of the ROM. Even if an EPROM is used, it takes a long time to rewrite the content and it is not possible to rewrite the content in real time during the PCM processing. Since a digital signal which is produced by converting an analog signal is subjected to a pulse-code modulation according to a given specific coding rule, there is no freedom of selecting an efficient coding rule in accordance with the digital signal.